The invention relates generally to an electrophotographic printing machine and, more particularly, to a process control system for carrier dispensing in a color electrophotographic printing machine.
Generally, an electrophotographic printing machine includes a photoconductive member which is charged to a substantially uniform potential to sensitize the surface thereof. The charged portion of the photoconductive member is exposed to an optical light pattern representing the document being produced. This records an electrostatic latent image on the photoconductive member corresponding to the informational areas contained within the document. After the electrostatic latent image is formed on the photoconductive member, the image is developed by bringing a developer material into proximal contact therewith.
Typically, the developer material comprises toner particles adhering triboelectrically to magnetic carrier granules. This two component mixture is brought into contact with the photoconductive surface. The toner particles are attracted from the carrier granules to the latent image. The carrier granules are then returned to the developer housing where they can be re-supplied with toner particles and where the new toner particles can be prepared with the appropriate tribo-electric charge. It is clear that the developer material is a critical component of the printing machine. Developer material has several critical properties including its electrical conductivity and its ability to properly charge toner. As the developer material ages its critical properties change, and when the material approaches the end of its useful life, copy quality deteriorates. The rate at which the various critical properties of the developer material change is dependent on both variable and fixed (for a given design) factors. The variable factors include the area coverage of the image(s) being developed, the relative humidity within the developer housing, and the temperature of the developer material. The fixed factors include the volume of developer material within the housing, the amount of work done to the developer material in transporting it through the toner resupply process, the amount of work done to the developer material in presenting the developer material to the photoreceptor, and other factors.
It has been found that by continuously adding additional new carrier granules to the developer housing, the rate of change of the developer material critical properties can be significantly reduced or eliminated. As additional carrier granules are added to the chamber storing the developer material, an approximately equal volume of developer material must be removed therefrom to maintain the developer material therein at the desired quantity. This removal of material is typically achieved by use of an overflow port that maintains a constant volume of material within the chamber. Therefore, a carrier replenishment system must provide regulation of the input of carrier granules into the development unit.
Prior embodiments of carrier replenishment systems mixed a fixed ratio of carrier with the resupply toner. Thus as the toner is re-supplied to the developer housing to make up for the toner developed on to the latent image, new carrier granules are also introduced into the housing. This approach provides a significant improvement over the case of no developer material replenishment. With a fixed toner to carrier ratio, the carrier replenishment rate is directly linked to the individual user's printing/copying area coverage usage and can easily vary from machine to machine. Thus, the fixed toner to carrier ratio approach does not necessarily provide the optimal rate of material replenishment, nor can it optimally compensate for the developer material aging rate changes due to variable factors such as area coverage, humidity, and/or temperature.
The above problem is more acute in a color system employing several developer units where small variations in the developability of an individual color become readily apparent as significant hue shifts in the final out output. It has been found that aging rate of developer material in each developer unit is different for each color, and that each color material has different sensitivities to the variable factors such as area coverage.